1. Field of Invention
This invention relates to a measuring device capable of measuring urging an ultra-high speed phenomenon at a high signal-to-noise (S/N) ratio by using an optical beam.
2. Description of the Prior Art
FIG. 1 shows a conventional measuring device comprising an SHG correlator for measurIng the width of an optical pulse by using the autocorrelation method. The repetitive pulses to be measured are halved by a half mirror 1 into two branches, the transmitted branch of which is reflected by mirror 2. On the other hand, the reflected branch has its advancing direction inverted by a corner cube 3 so that it is composed with the beam reflected by mirror 2. The beam thus composed is converged by an objective lens 4 to enter an SHG crystal 5.
SHG crystal 5 generates and outputs the secondary higher harmonic light of the incident beam. This secondary higher harmonic light is selected by a filter 6 so that its optical intensity is measured by a light receiving element 7. Thus, the pulse width of the beam can be measured by moving the corner cube 3 in the direction of the arrows to change the distance from the half mirror 1, i.e. the difference between the two optical path lengths.
A GaAs substrate has an electrooptic property wherein the polarizing plane of the reflected light is changed depending on the current flow. FlG. 2 shows a device for measuring the electric current flowing through a GaAs integrated circuit using such electrooptic properties. The output light from a YAG laser 8 is compressed by pulse compressing unit 9 to have a pulse width of picoseconds so that it is guided through a polarizer 10 and wave length plates 11 into a GaAs integrated circuit 12.
On the other hand, the reflected light passes through wave length plates 11 so that only light having a specific polarizing plane is selected by the polarizer 10 to enter a light receiving element 13. Light receiving element 13 converts the optical intensities of the incident light into electric signals. If the current to flow through the GaAs integrated circuit 12 changes, the polarizing plane of the reflected light is changed to change the intensities of the light incident upon light receiving element 13. In synchronism with the timing of the output light of YAG laser 8, a drive circuit 14 feeds an electric current to the GaAs integrated circuit 12. If the phase difference is shifted little by little, it is possible to measure the current to flow through GaAs circuit 12 and accordingly the operations of circuit 12. The measured results are displayed in display unit 15.
The device of FIG. 1, has certain problems. For example, because of the use of the autocorrelation method, the shape of the pulses cannot be measured. If the pulse is/of a different shape the pulse width is measured to be different. Moreover, since the secondary higher harmonic is generated by the SHG crystal, a weak light cannot be measured. Furthermore, although repetitive optical pulses can be measured, essentially coherent optical pulses cannot be measured.
On the other hand, the FIG. 2 device also has problems. For example, since the intensity of the light incident upon the light receiving element 13 is fine, it is difficult to increase the S/N ratio, hence signal processing, such as additive averaging, has to be executed. Moreover, a single phenomenon cannot be measured.